1. Field of The Invention
The present invention relates to an improved process which makes it possible to form a silicon-containing polycrystalline functional film on a substrate, which is eligible to use as a constituent semiconductor member of various semiconductor devices such as solar cells, scanning circuits of image-reading devices such as line photosensors, area photosensors, etc., thin film transistors (TFT), TFT arrays or matrixes used not only in operation circuits of liquid crystal displays but also in switching circuits of photosensors photosensitive devices. More particularly, the present invention relates to an improved process for forming a large area polycrystalline functional film containing silicon and germanium atoms by way of etching a thin film containing silicon and germanium atoms formed on a substrate so that a crystalline nucleus remains on said substrate and growing said crystalline nucleus.
2. Related Background Art
As for the method of forming a polycrystalline silicon film, there are known normal-pressure CVD method, low pressure CVD method (LP-CVD method), plasma CVD method, and the like. Among these methods, the low pressure CVD method has been widely used in order to form the polycrystalline silicon film. However, in the case of forming a polycrystalline silicon film by means of the low pressure CVD method, the substrate on which said polycrystalline silicon film is to be formed is required to maintain at elevated temperature during film formation and because of this, it is necessary to use a specific member of a high melting point such as quartz member as the substrate. Thus, there is a disadvantage for the low pressure CVD method that the polycrystalline silicon film product obtained unavoidably becomes costly.
In order to eliminate this disadvantage, there have been made various studies of electron cyclotron resonance plasma CVD method (ECR plasma CVD method), molecular beam epitaxy method (MBE method), and sputtering method in which a polycrystalline silicon film may be formed at a relatively low substrate temperature. However any of these methods is accompanied by unsolved disadvantages that a number of film-forming parameters required to be controlled upon film formation are present and it is difficult to stably and continuously form a large area polycrystalline silicon film having a uniform film property all over the entire of the substrate.
Other than the above, studies have been made of selective growth of a silicon single crystal as one of the silicon-on-insulator (SOI) techniques in order to develop a three dimensional integrated circuit. For instance, there has been proposed a method of repeating the cycle comprising crystal growth and film-etching in order to prevent formation of a crystalline nucleus on a SiO.sub.2 film in the low pressure CVD method with SiH.sub.2 Cl.sub.2 -HCl system (see, L. Jastrzebski et al., J. Electrochem. Soc. 130(1983), pp. 1571). This method belongs to an epitaxial growth method comprising forming a SiO.sub.2 film on a silicon single crystal, forming a window at the surface of the SiO.sub.2 film to expose the silicon single crystal, and growing single crystal utilizing the exposed single crystal silicon as a seed. Particularly, in view of causing a detardation time for a crystalline nucleus to be formed on the SiO.sub.2 film differently from the case where a crystalline nucleus is formed on a Si single crystal film, this method comprises alternately repeating step (i) of introducing SiH.sub.2 Cl.sub.2 gas to grow a Si crystalline nucleus on a SiO.sub.2 film within the detardation time and step (ii) of etching the Si crystalline nucleus formed on the SiO.sub.2 film using HCl gas instead of the SiH.sub.2 Cl.sub.2 gas, whereby forming a polycrystalline film. However there are disadvantages for this method that not only the time allocation of each of the repetition crystal growth and film-etching cycles but also the crystal growth temperature greatly influence to defect density of a crystal grown and it is extremely difficult to eliminate defects relative to twin, transfer defect, lamination defect, void, etc. even by properly controlling said time allocation and crystal growth temperature. There are also other disadvantages for this method that a relatively inexpensive ordinary substrate member such as glass plate cannot be used because the processing temperature is high and it is difficult to always form a polycrystalline silicon film containing grains of a large grain size with small grain boundaries without depending upon the kind of a substrate used.
Japanese Laid-open patent application Sho.63(1988)-44719 discloses a method of forming a polycrystalline silicon film at a relatively low substrate temperature. This method comprises separately introducing into a film-forming space, an active species (A) obtained from a silicon and halogen-containing compound and an active species (B) obtained from hydrogen gas or halogen gas, reacting said active species (A) and (B) to deposit a silicon film on a substrate placed in said film-forming space while introducing a gaseous substance having an etching property into said film-forming space to etch the surface of said film deposited on the substrate, whereby crystal growth in a given orientation is preferentially caused. Particularly, in the formation of a polycrystalline silicon film according to this method, there is used, as the substrate, a glass plate having a Si--N--H film thereon which is patterned with respect to its surface portions at which selective growth of a crystalline nucleus is to be caused. It is understood that a polycrystalline silicon film may be obtained at a relatively low substrate temperature according to this method. In the case of forming a polycrystalline silicon film on a commercially available inexpensive substrate such as glass plate according to this method, an insulating film such as Si--N--H film is firstly formed on such substrate (glass plate), the insulating film (Si--N--H film) formed on the substrate is subjected to patterning to provide patterns at a predetermined interval at the surface thereof, and formation of a polycrystalline silicon film is performed while selectively growing crystalline nucleuses formed at those patterns. The patterning in this case includes application of a resist, mask adjustment, exposure, development and removal of the resist. Thus, it is extremely difficult to stably and repeatedly form a high quality polycrystalline silicon film at an industrial scale according to this method, since there are often caused problems relative insufficient development, residue of the resist, uneven etching, formation of undercut portions, and the like which lead to increase crystalline defects for the film formed.
Other than the above, International Publication No. WO90/12126 discloses a method of forming a polycrystalline silicon film on a non-single crystal substrate such as glass plate by chemical vapor deposition, comprising the steps of exciting hydrogen gas by the action of an activation energy to generate active species (H) in a space other than a film-forming space of a film-forming chamber containing the substrate therein, introducing the active species (H) and a silicon-containing film-forming raw material gas into the film-forming space at the same time but separately from each other, bringing the active species (H) into contact with the film-forming gas by mixing them to produce a plasma region in the film-forming space kept at a predetermined pressure, and periodically changing the concentration of the active species (H) near the surface of the substrate maintained at a predetermined temperature, whereby causing said polycrystalline silicon film on the substrate. This method makes it possible to form a polycrystalline silicon film having a good orientation at a relatively low substrate temperature. However, there is still a disadvantage for this method that it is difficult to provide uniform distribution of the concentration of the active species (H) allover the entire surface of the substrate and because of this, a well-skilled technique is required in order to form a uniform polycrystalline silicon film all over the entire surface of a large area substrate. In addition, the crystal grain size of the polycrystalline silicon film obtained according to this method is at most of about 5000 .ANG..
In view of what above described, there is a demand for provision of an improved method which makes it possible to stably and repeatedly form a high quality large area polycrystalline silicon film containing crystal grains of a large size in a simple manner.